The invention relates generally to level gauging, and more specifically the invention relates to a method and an apparatus for radar-based level gauging.
Radar-based methods are extensively used for level gauging, i.e. measuring a distance from the top of a tank to a surface of a liquid or some kind of granular solid stored in the tank, by means of transmitting a microwaves towards the surface of the liquid or the granular solid, receiving the microwaves as reflected against the surface of the liquid or solid, and calculating the level of the liquid or solid in the tank from the propagation time of the transmitted and reflected microwaves.
One very general problem in this respect is that the tank typically includes various structures, such as support beams, pipes, agitators, tank walls, etc. Such structures may also reflect microwaves, which can interfere with the microwaves reflected from the surface of the liquid or solid, the level of which being gauged. Many liquids are comprised of oil and petroleum products, which have a rather low dielectric constant, i.e. below 3, which makes the microwave echo from a smooth surface about 12-20 dB weaker than an echo from a metallic surface. If the surface is turbulent the signal strength of the echo will be further reduced, i.e. about 6-20 dB weaker depending on antenna size, and as a conclusion an echo from a geometrically small metallic structure can easily be stronger than the echo from a turbulent oil surface.
One method which has been used for a long time in radar level gauging is to mechanically incline the antenna slightly to suppress echoes from disturbing objects more than the echo from the surface, i.e. to improve signal-to-disturbance ratio.
The selection of correct microwave echo is thus extremely important and any possibility of distinguishing microwave signals reflected from the surface of the liquid or solid from microwaves reflected from other structures is very useful.
Typically, prior art radar level gauges typically select the strongest echo as detected within the antenna beam.
The ideal case is to use an antenna with a rather narrow lobe located in the tank where no disturbing echoes are close to the antenna lobe. In this case the surface echo may be the strongest one even after some degradations due to turbulence, foam etc. For smaller tanks various tank structures may approach the antenna beam, not at least since the antenna has to be smaller. Furthermore when the echo from the surface is close to a disturbing echo there is a possibility for a large measuring error.
A main object of the invention is thus to provide a method and an apparatus for radar-based level gauging, wherein detected microwaves as reflected from the surface of the liquid or solid can be distinguished from detected microwaves as reflected from other disturbing structures.
In this respect there is a particular object of the invention to provide such a method and such an apparatus, which are very useful in tanks having a large number of disturbing structures and in tanks where the radar-based level gauging equipment has to be mounted in regions where disturbing structures do occur.
A further object of the present invention is to provide such a method and such an apparatus, which provide for level gauging also of highly turbulent surfaces, where the reflected microwaves are weak.
A still further object of the invention is to provide such a method and such an apparatus, which are reliable, efficient, accurate, and precise.
A yet further object is to provide such a method and such an apparatus, wherein a dynamic cancellation algorithm is formed in order to strongly decrease the influence of disturbing echoes and thus increase the accuracy of the level measurement.
These objects, among others, are attained by methods and apparatuses as claimed in the appended claims.
According to a first aspect of the present invention there is provided a method for radar-based gauging of the level of a substance, e.g. a liquid or a granular solid, in a tank having at least one interfering structure, such as e.g. a beam, an agitator, or a tank side wall. The method comprises transmitting a microwave signal in a plurality of differently directed radiation lobes, e.g. two or four lobes, where each radiation lobe is directed towards the surface of the substance and at least one of the radiation lobes is directed towards the interfering structure. Typically, there may be a number of interfering structures in each radiation lobe.
For each of the radiation lobes, the microwave signal as reflected against the surface of the substance is detected and so is the microwave signal as reflected against any interfering structure(s). Thereafter, based on signal strengths of the detected microwave signals, the detected microwave signals, which have been reflected against the surface of the substance, are distinguished.
Finally, based on a propagation time of at least one of the microwave signals distinguished as those, which have been reflected against the surface of the substance, the level of the substance in the tank is calculated.
Preferably, the detected microwave signals, which have been reflected against the surface of said substance, are distinguished by means of their similar signal strengths, optionally after correction for the inclination of the respective lobes. It is well known that a surface echo from a vertical radiation lobe is stronger than a surface echo from an inclined radiation lobe. Thus, these signal strength differences originating from inclination angles of the radiation lobes may be compensated for.
Still preferably, the differently directed radiation lobes are separated by about 0.5-1 times the radiation lobe width of any of the plurality of differently directed radiation lobes, wherein one of the radiation lobes can be essentially vertical. Typically, the widths of the radiation lobes are similar. By such separation the lobes can be arranged to have small coupling, which is a prerequisite for independent function of the lobes. Depending on the particular application some radiation lobes may be more inclined than others.
In a typical implementation of the present invention four differently directed radiation lobes are produced, all of which having the same inclination. A surface echo will have about the same amplitude in each of these four lobes, while a disturbing echo will have very different amplitudes in the four radiation lobes and the disturbing echo in the lobe(s) pointing most away from the disturbing structure will have very low amplitude. Thus this method will not only provide a diagnostic method to distinguish echoes originating from the surface to be gauged from those originating from fixed disturbing structure(s), but also provide means for selecting the optimum radiation lobe, i.e. the lobe having highest signal-to-disturbance ratio, for subsequent use. Obviously, different radiation lobes may be optimum for different tank environments and different levels of the gauged surface
Antenna devices capable of producing multi-lobe microwave radiation and of receiving reflected microwave radiation in each lobe separately include parabolic antennas fed by multiple horns, or planar antennas with suitable feeding networks. Two preferred planar antennas are patch arrays fed by Butler or Blass matrices.
According to a second aspect of the present invention there is provided a radar-based level gauge apparatus for performing the method according to the first aspect of the invention.
By means of the present invention a very robust routine for distinguishing detected microwave signals, which have been reflected against the surface of the substance gauged, may be implemented. As compared to prior art devices the invention provides for measurement in more disturbing environments, i.e. where more interfering echoes do occur, with higher accuracy. For instance, microwave signals reflected at the surface of the substance, may be distinguished despite being weaker to much weaker than a microwave signal as reflected against an interfering structure.
The radar-based level gauges are used to measure levels in tanks, which for the purpose of the present invention include not only large containers but also processing apparatuses such as, for example, reactors, centrifuges, mixers, hoppers, graders, or heat-treatment furnaces and similar devices, which are used in e.g. food chemistry, pharmaceutical chemistry, biochemistry, gene chemistry and petrochemistry.
Further characteristics of the invention, and advantages thereof, will be evident from the detailed description of preferred embodiments of the present invention given hereinafter and the accompanying FIGS. 1-4, which are given by way of illustration only, and thus are not limitative of the present invention.